Electronic musical instrument



April 13, 1965 R. H. PETERSON 3,178,499

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INVENTOR. RICHARD u. PETERSON ATTORNEY April 13, 1965 R. H. PETERSONELECTRONIC MUSICAL INSTRUMENT 3 Sheets-Sheet 2 Filed May 29. 1961 lcmsmmvrm RICHARD H. PETERSON i420 ATTORNEY PM 3997f April 13, 1965 R. H.PETERSON 3,178,499

ELECTRONI C MUS I CAL INS TRUMEN T Filed May 29. 1961 5 Sheets-Sheet 5 g2 710 i fly- 7691: 7% 762 @725 GO (2 0 122 0 T (I? (I;

' INVENTOR. RICHARD H. PETERSON ATTORNEY United States Patent 3,178, 39ELECTRONIC MUSICAL INSTRUMENT Richard H. Peterson, 10108 Hamerv Road E,

v Uaklawn, Ill. Filed May 29, 1961, Ser. No. 1133559 Claims. (Cl. 84-131) This invention relates to electronic musical instruments. Morespecifically, it relates to improved envelop'e-controlling keyingattenuators for use with organs having tone generators that are incontinuous oscillation. In organs of this type, the playing keys or" theinstrument control the switching of signals from the oscillators intothe amplification circuits. Some organs use ordinary mechanical switchesfor this purpose, with the result that the starting and stopping of thetone occurs in a very abrupt, unmusical manner. Mechanical switchesemployin'g variable impedance elements are known, but these are at bestonly partial solutions of the problem, mainly because they cannot secureaccuracy in duplicating the attack and decay characteristics of acousticdevices.

The tone of an organ pipe, for example, takes a short but verysignificant amount of time to decay after the wind to the pipe has beencut off. The resultant lingering of the tone and the mixing of this tailwith the notes that follow is highly desirable. Electronic switchingdevices, or electronic gates, in the form of vacuum tube or transistorkeyers, or diode attenuators are known to have been used for the purposeof envelope control in musical instruments. All such devices known toapplicant, however, fail in one or more important respects to meet allof the requirements that are desirable in such devices. For a keyinggate to be completely satisfactory, it must be low in cost, sincegenerally a great number must be used. Usually, where unification and/or octive coupling and intermanual coupling are provided, it isnecessary to have several gates associated with each playing key on theorgan. The gates must also have controllable attack and decaycharacteristics so that the attack and decay of difierent types ofacoustic instruments can be simulated. Further, the gate must notintroduce distortion of a type that would make the harmonic structure ofthe tone passing through it change in an unnatural manner during theattack or decay period. Also, the operation of the gating circuit mustnot introduce undesirable transients, such as clicks or thumps. Finally,it is essential that leakage through all the gates that are closed atany particular instant must be so small as to produce no noticeableeffect.

It is therefore the object of this invention to provide improvedenvelope-controlling gating circuits having all the desirablecharacteristics enumerated. It is a further object to provide gatingcircuits that also distort the tone in a desirable manner such that theharamonic structure as well as the speech characteristics closelyduplicate ce tain acoustic instruments. It is a still further object toprovide gating circuits so that regular or percussion type musicalinstruments may be duplicated.

In the annexed drawings:

FIGURE 1 is a schematic diagram of one type of oscillator that can beused with the invention, together with a block representation of otherparts of an organ system;

FIGURE 2 is a schematic diagram of a gate having a desirable linearrelationship between attenuation and applied voltage;

FIGURE 3 is a schematic diagram of a gate capable of producing a widevariety of attack and decay characteristics;

FIGURE 4 is a schematic diagram of a gate requiring 'ice relativelylower operating current than those of FIGURES 2 and 3;

FIGURES 5, 6 and 7 are schematic diagrams of gates having threedififerent kinds of desirable tonal characteristics;

FIGURE 8 is a graph or" air pressure and tone amplitude as a function oftime for a conventional organ pipe;

FIGURE 9 is a schematic diagram of a gate in which a single attenuatorsection utilizes two diodes; and,

FIGURE 10 is a partial schematic diagram indicating a modification ofFIGURE 3.

In the embodiment selected to illustrate the invention, and referringfirst to FIGURE 1, transistor 10d, transformer lldl, and timingcomponents 193, and 194, together with a power source 192, form anoscillatory circuit that will produce a signal at a frequency determinedprimarily by the constants of resistor 193 and capacitor 1%. This is oneform of blocking oscillator circuit and is representative of one type ofoscillation generator that is suitable for use with the gating circuitsof the invention. Since oscillators of this general type are known tothose skilled in the art, this application is not encumbered withdetails of operation. In a complete organ there is generally a similarcircuit provided for each semi-tone throughout the gamut of theinstrument. Since these oscillators are not very stable in themselves,they are customarily synchronized in chains, or cascades, controlled bya master oscillator. Capacitors 195 and 1% are coupling capacitors forcoupling synchronizing signals into and out of the oscillator 99. Forexample, capacitor 1% might be connected to a stable oscillator oneoctave above the frequency of oscillation of transistor Itltl, andcapacitor 1% couples energy from the oscillator 99 or" FIGURE 1 into aconductor for activating the next cascade oscillator (not shown)operating one octave lower.

During operation, a signal voltage in the form of a sawtooth waveappears across resistor 103 and capacitor we and this signal is appliedto the input terminal 119 of the gating circuits .197, of which onlythree are illustrated. Each gating circuit may be any of the gatingcircuits illustrated schematically in FIGURES 2, 3, 4, 5, 6 and 7.Signal, after passing through a gating circuit, is applied toconventional amplification and filtering circuits represented by therectangle 108 and is then translated into sound by the loudspeaker 109.The gating ci cuits according to the invention are diode type attenuatorcircuits wherein the attenuation of the circuit is a function of a DC.current which is controlled by a playing-key-operated switch, such asthat represented at 111.

FIGURE 2 is a keying attenuator or gate capable of a very greatattenuation ratio. In addition, the amount of attenuation is relativelyindependent of diode characteristics and is relate in a very linearmanner to the current flowing in the circuit. This makes possible theuse of this gate in circuits arranged such that the gates can beusefully employed under conditions Where they are only partially open.Input to the gate is applied across terminals All and ground 1.20.

Resistor 201 and diode .216 form a signal voltage divider whereby thesignal appearing at point 221 is substantially that appearing atterminal lit) multiplied by a quotient consisting of the dynamicimpedance of the diode 2116 divided by the sum of that same dynamicimpedance plus the impedance of resistor 291. This can be written:

R V22 equals V110 UIIIIBS wherein R is the dynamic impedance of diode216.

arran e The dynamic resistance of diode 216 is a function of the currentthrough the diode such that, with no current, the diode impedance isextremely high and with typical diodes may be in the order of megohms ormore. If a current is caused to flow through the diode, however, itsdynamic impedance will become very low, typically in the order of 50ohms. In this circuit the DC. current path is from the bias power source2115 through resistor 299, resistor 2%, resistor 22%, then through diode21d, and back to ground 12G. Conductor 23%! extends from resistor 208 toresistor 285, and has three lateral connections to capacitor 212 andresistors 21 and 266.

It is apparent, then, that the signal appearing across diode 216 will begreatly attenuated when a bias current is flowing. However, the ear issensitive to an enormous range of intensity levels, the ears sensitivitybeing according to a logarithmic curve. This means that the attenuationobtained in a gate employing a single diode may not be sutficient toavoid an objectionable amount of residual leakage through the closedgate. For this reason, a second attenuator section consisting ofresistor 2il2 and diode 217 is provided, and bias current is supplied todiode 217 through the resistor 2%. For very critical applications, stilla third attenuator section may be provided as by resistor 263 and diode21%, receiving current through resistor 207. The output of the attenuator is connected through resistor 2494 to the bus 226 which may becommon to similar gates associated with other oscillators.

In this attenuator, the total attenuation is equal to the product of theattenuations aiiorded by each of the three sections. It should be notedthat the resistances of resistors 201, 292, 203 and 204 are relativelylow compared to the resistances of resistors 2%, 2% and 2&7, andtherefore do not unreasonably reduce the signal transmission through theattenuator during the times when the diode impedances are high. Switch213 is a conventional key switch arranged to be closed by the depressionof one of the playing keys of the organ. When this switch is closed, thejunction between adjustable resistors 208 and 299 is shorted to groundand as a result no current can flow from the bias supply 215 through thediodes.

However, upon closure of switch 213, bias current through the diodesdoes not cease instantly because of the energy that has been stored incapacitor 212, which, prior to the closure of switch 213, was connectedacross bias source 215 through resistors 2% and 2 39. Upon the operationof switch 213, capacitor 212 discharges through resistors Zfld, 2%, and207 in parallel, but each in series with its respective diode 216, 217,and 218. It can also discharge through the resistor and switch 213.Since resistor 2% is ordinarily of a much smaller value than theresistors 2%, 206 and 2W7, the discharge time of capacitor 212 isprimarily determined by the resistance of resistor 20%. The values ofresistor 208 and capacitor 212 therefore control the abruptness withwhich the bias current ceases to flow, and the rate of attack of thetone signal transmitted through the gate.

Upon opening key switch 213, bias current is again allowed to flowthrough the diodes but this current will not build up instantly becauseof capacitor 212. The build-up of voltage at point 225 is determined bythe time constant of capacitor 212 and the combined resistance ofresistors 2% and 209. Since resistor 2ti9 is ordinarily many timesgreater in resistance than resistor 2%, the resistance of resistor 269substantially determines the build-up of current and therefore the decayrate of the signal through the attenuator.

At 214 I have illustrated an additional key switch which may typicallybe controlled by a difierent playing key than that which controls switch213. This second playing key may be one of theplaying keys on a separateplaying manual or it may be related by an octave or other musicalinterval to switch 213 on the same keyboard.

One terminal of switch 214 is connected to the junction betweenresistors 208 and 269 as before, while the other terminal is connectedto a resistor 211 which resistor is connected to the slide contact onthe potentiometer 219, connected across the power source 215, to avoid ashort circuit of part of the source 215 if both switches 213 and 214happen to be closed at the same time. If the slide on resistor 21b isconnected in the position illustrated by the dotted line, operation ofthe switch 214 will be substantially as that of switch 213. However, ifa connection to resistor 210 is made as shown by the full line arrow,the junction of resistors 2b? and 209 will be connected to a DC. voltagewhen switch 214 is open. This results in partial attenuation of thesignal through the gate, the degree of attenuation being determined bythe position of the slide on the potentionmeter 216.

Thus it is possible to control the same gate from two differentkeyboards, and have one keyboard cause the signal to be transmitted withsubstantially no attenuation while the other keyboard will cause atransmission of a partially attenuated signal. Typically, potentiometer21b is common to a great many key switches such as that shown in 214 anda corresponding number of separate attenuators corresponding to thedifferent notes of the scale, with the result that the adjustablepotentiometer 21% is, in effect, a balancer that can balance theloudness of all the signals controlled by switches 21?; and all thesignals controlled by switches 214. All the keys 213 secure fullloudness, but all the keys 214 secure partial loudness, and the signalpotentiometer may be adjusted during play by the player, to change therelative loudness for all the notes controlled.

In FIGURE 3 I have shown a gate circuit similar to that of FiGURE 2, butwith only two diode attenuator sections instead of three. Whether twosections or three are employed in any instance depends primarily on theattenuation ratio desired and on the desired shape of decay curve withrespect to time. In FIGURE 3, the attenuation of the gate will be afunction of the square of the current in resistor 308, while in the caseof the attenuator shown in FIGURE 2, the attenuator will be a functionof the cube of the current in the corresponding resistor 2%. Thefunctioning of the attenuator sections shown in FIGURE 3 is the same asthe corresponding parts in FIGURE 2. Key switch 313 (FIGURE 3) operatesin the same manner as key switch 213 (FIGURE 2).

But key switch 335 which operates by grounding point 38 9, performs afunction different from that of key switch 214. Associated with keyswitch 335 is capacitor 34-6), resistors 341 and 3 54, diodes 342 and343, and the potentiometer 35o connected across bias power source Whenkey switches 31?} and 535 are both open, a current will flow from biaspower source 335, through resistors 369, 308, conductor 330, and throughthe parallel paths consisting of resistors 305 and 3% and the diodes 31dand 317 and back to the power source. At the same time capacitor 312will have charged through resistors 398 and 399, and capacitor 340 willhave charged through resistor 399 and diode 343. Upon closing switch335, capacitor 312 will discharge through resister 3% and diode 343, andcapacitor 340 will discharge through resistor 341. The rate of openingthe gate will thus be determined by resistors 3638 and $41. Upon openingkey switch 335, current in conductor ass will again build up, but at arate determined by the charging times of capacitors 312 and 340.Capacitor 340 also has an ancillary charging circuit including resistor3 54 and diode 342. If the voltage at terminal 348 is greater than thevoltage to which capacitor 340 has charged, the polarity of the voltageacross diode 342 will be such as to allow capacitor 340 to draw currentfrom the power source 315, through potentiometer 35b, resistor 344 anddiode 342. If the slider of potentiometer 35b is connected to the top ofthe resistance element, the ancillary charging path will be effectiveduring the entire charging cycle of capacitor 340, while if the slideris connected to the bottom of the resistance element (ground) theancillary path will not be efiective during any part of the chargingcycle. Thus we have provided a keying gate that can be operated from twodilferent key switches wherein each switch causes the gate to open andclose at a different rate with respect to time. Further the rate ofopening and closing of the gate as con trolled by switch 335 can beadjusted over a wide range by purely electronic means so as to duplicatethe envelope characteristics of an equally wide range of instruments,from organ pipes to percussion instruments. It should be understood thatthe bias power source 315 and the potentiometer 350 can be common to agreat many gates, such as all of those associated with the switches inthe bank, or playing key board, including wey switch 335.

The envelope characteristics of certain percussion instruments, such asthe piano, can be even more closely duplicated if resistors 2&9, Ell),499 etc., are voltagesensitive resistors, such as silicon carbidevaristors. Such resistors display the characteristic of having differentresistance values depending upon the amount of current flowing throughthem. Since the current flowing through resistor 30? varies during thecharging and discharging cycles of capacitors 312 and 34% it can be seenthat the shape of the charging and discharging curves with respect totime will be altered if the resistance is voltage or current snesitive.Generally, the effect on the decay characteristic of the tone passingthrough the gate is to cause the initial portion of the decay curve todecay more quickly than it would otherwise, and the latter portion moreslowly.

In all of the gates thus far disclosed it is advantageous that the inputsignal to the gate be a pulse (positive with the diodes connected asshown), rather than an alternating signal having positive and negativeexcursions. The oscillator of FIGURE 1 delivers such a pulse. his isbecause a signal that causes a reversed voltage to appear across thediode (such as for example diode 216 in FIGURE 2) will cause the diodeto conduct during that portion of the signal cycle and hence cause someundesired attenuation when the gate is open.

Some diodes, particularly silicon diodes, require a certain definiteforward voltage before any substantial conduction is established. Withsilicon diodes this voltage is usually about 0.4 volt. With such diodesit is allowable for the signal to have a negative excursion, providingthe absolute magnitude of the same is less than 0.4 volt.

Where silicon (or equivalent) diodes are used, and where the signalvoltage into the gate is low, it is possible to use the gate circuitshown in FIGURE 4. This is a two stage attenuator consisting of a firstattenuator section comprising resistor 4d]. and diode 416, and a secondattenuator section comprising resistor 4G2 and diode 417. Each of theseattenuator sections is an audio-frequency voltage divider wherein thediode represents a variable impedance whose value depends upon the DC.current through the diode. In FIGURE 4, the source of this diode currentis shown at 415. With respect to this DC. current the diodes are inseries, the complete circuit including source 415, resistors ass, M8 and492 as well as the diodes 416 and 417. With respect to signal voltageshowever, the diodes are in a parallel relationship because capacitor 412is a low impedance to signal frequencies. As in the previous circuits, akey switch 413 controls the DC. current through the diodes, and the rateof buildup and decay of this current is a function of the time constantsof the resistors 4% and 4&9 and capacitor 412. Note that when the DC.current is not flowing through the diodes due to switch 413 beingclosed, both diodes are in their high impedance condition.

A signal Voltage of either polarity however will tend to cause one diodeor the other to be biased in the sense to cause it to conduct andtherefore attenuate the signal. For the reasons previously mentionedthis will not occur 6 provided that the magnitude of the signal does notexceed the forward threshold voltage that is necessary to establishconduction in the diode. With this gate it is desirable for the signalto be a true A.C. signal having both positive and negative excursions,and therefore an input coupling capacitor is shown at 440. As comparedwith the gates of FIGURES 2 and 3, the gate of FIGURE 4 uses a singleDC. current to control both attenuator sections, and the size or"capacitor 412 is correspondingly reduced.

An important advantage of all of the keying gates thus far disclosed isthat during the attack and decay period, tone signals passing throughthe gates are not subject to an excessive or objectionable harmonicdistortion that would cause the character of the tone to change.Furthermore the tones decay in a very smooth manner down toinaudibility.

Certain acoustic instruments, particularly organ reed pipes, however,havethe peculiar characteristic that their intensity envelopes are asshown in FIGURE 8. These pipes will not sound at all below windpressures that are below a certain minimum pressure. This results in anattack that is both abrupt and somewhat percussive in character and in arather abrupt decay. In no case however is there a transient, or anabruptness of the type associated with ordinary mechanical signalswitching. Furthermore, the harmonic structure of reed pipes isgenerally very ditferent from the tone signals produced by ordinaryelectric oscillators in that in the reed pipes a great deal of energy isconcentrated in the low order harmonies of the tone. Electrical waveforms on the other hand almost always have too much fundamental ascompared with the first few low order harmonics, and they generally havemuch too much high order harmonic energy.

In FIGURES 5, 6 and 7, I have shown keying gates that are capable ofproducing reed-like intensity envelopes. In addition these gates alsointroduce distortion of a type that is highly desirable and that resultsin the production of tones of unusual beauty and that could nototherwise be produced without resorting to complicated filtering systemsemploying many times the number of components required according to theinvention.

Referring first to FIGURE 5, resistor Sill and diode 53.6 comprise anattenuator section of the type already described. This may be thought ofas a shunt attenuator, because the diode, when conducting, shunts thesignal path to ground. A second attenuator section includes diode 561and resistor 525. This is a series attenuator because the variableimpedance element (diode 561) is in series with the signal path. DC.current for diode 516 flows in the circuit including the source 515 andresistors 5&5, 503 and 509. As before, a key switch (513) controls theapplication of the bias current and resistor 598 and 5%? together withcapacitor 512 control the attack and decay characteristics. When biascurrent is flowing a voltage will appear at point 550 that is equal tothe forward voltage drop across diode 516. Since this voltage is of theopposite polarity necessary to establish conduction in diode 561, diode561 will be open and no signal appears on output bus 526.

Closing key switch 513 cuts off the bias current through diode 516 andtherefore removes the negative voltage from point-550. Now positivepulses of signal from the oscillator in FIGURE 1 (or equivalent), chargecapacitor 566 (which is across diode 516) through resistor 561, andterminal 55% comes to a potential allowing conduction of the signalthrough diode 561to the output 'bus 520. Because the voltage acrosscapacitor 569' changes dynamically during each signal cycle, theimpedance of diode 561 also varies dynamically, which results indistortion of the signal. This distortion prevents extremely sharpdiscontinuities in the wave form that appears on the output bus, and istherefore substantially equivalent to a very effective low pass filter.As a result the output tone is free from the excessive high orderharmonic enai /sues ergy that is present in most electronically signals.

Varying the values of resistor &1 and capacitor 560 makes possible awide variety of desirable sounds. Increasing the value of capacitor seefor example, lowers the frequency at which the circuit operation beginsto eliminate higher order harmonics. It is interesting that thefiltering action of the diode circuit does not reduce the low orderharmonics and it is this selective filtering action that results in theoutput signal having the highly desirable harmonic energy distribution.In addition, the voltage across capacitor 560 that is due to the signalpulses, builds up during the attack period and decays during the decayperiod, in a manner that causes the gate to produce keyed signals havingenvelope characteristics as shown in FIGURE 8.

Gates according to FIGURES 6 and 7 operate in the same manner as that inFIGURE 5, except for the inclusion of an inductor in the circuit. Signalpulses through inductor 662 (FIGURE 6) cause ringing or the productionof damped oscillations at a resonant frequency determined by theconstants of capacitor use and inductor 662. This resonance is usuallyadjusted to be an octave, or up to several octaves, higher in frequencythan the frequency of the signal. This formant effect is exactlyanalagous to the effect that an acoustic resonator has on the tone of anorgan pipe. To carry the analogy further, the basic electrical signal,as modified by the distortion of the gate as previously described, isthe equivalent of the pulse of air produced by a vibrating reed, whilethe ringing of the formant circuit duplicates the function of theresonator.

FIGURE 7 operates in a similar manner except that the inductor isconnected from the output terminal 7% (of diode 761) to ground, and theoutput signal is conpied to two output busses 720 and 770. Variousmethods coupling the output of the gate into the amplification circuitsresult in ditfering proportions of the basic signal and the ringingfrequency signal, to be received by the amplifier. The basic signal inFIGURE 6 is coupled through the inductor 662, and this results inrelatively less high frequency energy being received by bus 621}. InFIGURE 7 one bus 770 is coupled through capacitor 7'71, but bus 729 iscoupled through resistor 782.

In FIGURE 8 the full line curve is a graph of air pressure delivered tothe slot of the organ tube, and the dotted line represents the intensityof the sound emanating from the tube. The x axis represents time. Forthe full line, the y axis is pressure, and for the dotted line the yaxis is loudness.

At the outset the pressure rises and air flow lags only a little behind.But until a certain degree of turbulence is generated at the slit, themain air column will not start vibrating. At time T vibration starts,and passes the lower level of audibility at time T reaching a volume atT that would continue if the pressure did not continue to rise. At pointT the pressure is at its 100 percent plateau and at time T the vibrationis also at 100 percent.

This continues indefinitely until the player lets the key up at time TOn the way down the sound volume slightly exceeds what the instantaneouspressure would sustain in a steady state. At point T (which is aslightly lower intensity than at point T the air flow at the slit failsto maintain any energy supply to the main column. The decrease is rapiddown to inaudibility at T and at T the most sensitive measuring meanscan no longer detect oscillations in the main tube. Finally at T thelast vestige of movement of air at the slit ceases.

The foregoing outline is believed to be substantially correct, but thepragmatic fact is that with gates according to any one of FIGURES 5, 6,and 7, envelopes can be secured so closely similar to the dotted line ofFIGURE 8 that even a highly cultivated musical car can generated hardlydistinguish between the electronically produced and the acousticallyproduced music.

In FIGURE 9 the resistor 901 and diode 981 function as a firstsignal-voltage divider, the delivered signal from which enters a singlesection with current-sensitive impedances in the nature of diodes 961and 916. For many not too exacting requirements, the mere omission ofdiode 9811 lets the original signal go directly into the doublediodesection. In either instance, the signal at terminal 982 enters the samereceiving gate.

The application of control current causes the shunt diode 916 to assumea high impedance state. Current is fed to the shunt diode 916 throughresistor 97%. When current is flowing in this circuit, with key switch913 open, terminal 9% will assume a potential equal to the forwardvoltage drop of diode M6, or about 0.4 volt. This is the cathodepotential of diode 961.

The anode of diode 961 is at a considerably higher negative potential,the exact potential being determined by the values of resistors 909,9618, 905 and 9%91, together with the source resistance across terminalsand 1241. Since this biases the diode 9%]. in reverse, its impedance ishigh, and substantially no signal arises at point 9%.

When the control current is removed, by closing key switch 913, thepolarity across both diodes 961 and 981 is reversed because the inputsignal is a positive pulse. Under these conditions diode 961 conducts,and diode 916 opens. The signal now passes on through conductive diode$61, to point 9%, coupling resistor 977, to the output bus 9'21), andappears across resistor 925.

Resistors 908 and 9119 and capacitor 912 control the attack and decay asdescribed previously, in connection with resistors 2% and 2439.

Either or both of the capacitors use or 978 can be employed to vary thetone quality, as described in connection with the gate of FIGURE 5. Inaddition, these capacitors, particularly 966, increase the attenuationratio by eliminating high order harmonics that increase the peakamplitude of the signal into the two-diode attenuator.

To complete the full duplication of the envelope characteristics thepiano has when the usual sustain pedal is being used, any of the playingkeys 213, 214, 313, 335, etc., may be operatively associated with themomentary impulse unit of FIGURE 1. I have indicated the key 335connected in this way. Ilustrated is a portion of FIG- URE 3, modifiedby adding the impulse unit 352, having a jumping armature 3-54 adaptedto overshoot and make momentary contact with terminals 356 and 358 overa very short time interval that can be adjusted with high precision.

Closure of switch 335 energizes the winding 366 and causes the armature354- to jump into momentary contact and connect point 39h to ground,just as in FIGURE 3, but only for the predetermined short time interval,independent of how long it takes the player to get the switch 335 openagain.

Later, when the switch 335 is opened by the player, it is desirable thatthe sound terminate with the same envelope as a piano string whenengaged by its felt damper. The companion switch 335-1 is mechanicallyconnected to switch 335 to open when switch 335 is closed and close when335 is opened. T he return of the parts to full line position restoresan electronic damping action, through resistors 3&2 and 3118. Resistor3119 has a high impedance to secure a slow, percussion decay while key335 is still held down by the player, but resistor 362 has only afraction of that impedance, proportioned to secure the much shorterdecay characteristic of the felt-damped piano string.

To give the player a sustaining control, equivalent to a pianosustaining pedal, switch 364 is provided in series with switch 335-1.This switch is opened by the sustaining pedal to secure the sustainingaction, in which case closure of switch 335-1 has no eiiect, and thetone continues to decay slowly. But if the player, with switch 364 i)open, plays a chord or a succession of notes, and lets all the keyswitches open again, and subsequently closes switch 3M, the simultaneousrapid decay of all the notes still sounding gives a performancesubstantially indistinguishable from that of a piano. The electronicsustaining pedal operates a gang switch which includes a switch 364- forevery note.

it will be understood that each signal source is operatively associatedwith a number of gates, and that playercontrolled stop means to selectthe gates that can transmit to the amplifier and filter 198 are alwaysprovided. These are usually part of the filter system itself, but foradditional ciearness, they are individually indicated in FIG- 1 at S1,S2, and S3.

Others may readily adapt the invention for use under various conditionsof service by employing one or more of the novel features disclosed orequivalents thereof. As at present advised, with respect to the apparentscope of my invention, I desire to claim the following subject matter:

1. in an electronic organ, in combination: a source of audio-frequencyelectronic signal; a gating circuit; said gating circuit comprising afirst attenuator section comprising a first signal-voltage divider; saiddivider comprising a relatively constant impedance and acurrent-sensitive variable impedance; said divider having an inputterminal connected to receive signal from said source; a connection forreceiving a first derived signal from across said variable impedance; asecond attenuator section comprising a second similar signal-voltagedivider connected to receive said first derived signal, and comprising arela tively constant impedance and a variable impedance; 2. secondconnection for receiving a second derived signal from across the variale impedance of said second signalvoltage divider; transducing meansconnected to receive said second derive/d. signal and generate soundcorresponding thereto; and direct current biasing means operativelyassociated with said variable impedances for changing the condition ofsaid variable impedances from high to low and vice versa.

2. A combination according to claim 1 in combination with a third,generally similar, attenuator section, interpolated between said secondsignal-voltage divider and said transducing means.

3. A combination according to claim 1 in which both variable imped-ancesare diot es; each diode having a low impedance value many times smallerand a high impedance value many times greater than the impedance of itsassociated relatively constant impedance.

4. A combination according to claim 3, in which said diodes are adaptedto change from low to high impedance and vice versa at a predeterminedpotential diiiering substantially from zero; whereby oscillationsinvolving excursions in both directions, but not exceeding saidpredetermined potential in the direction from. zero to saidpredetermined potential, may pass through without distortion.

5. A combination according to claim 3 in combination with playing keycontrolled means operatively associated With said attenuators fordelivering direct current potential simultaneously to the diodes of bothattenuators to change the impedance of both diodes.

6. A combination according to claim 5 in which each attenuator has itsown independently predetermined timeattenuation ratio curve in responseto the receipt of any stated direct current potential; whereby theultimate attack curve of the inception of signal delivered to saidtransducing means is a function of the time-voltage curve of the keycontrolled direct current, compounded with the time-ratio curves of thefirst attenuator for various direct current voltages, combined with thetime-ratio curves of the second attenuator for various direct currentvoltages.

7. A combination according to claim 6, in combination with a thirdattenuator connected between said transducing means and one of theoutput connections of said second attenuator; and biasing meansoperatively associated til with said attenuators for biasing the diodesof all three of said attenuators to substantially confine signal tochannels leading to said translating means, or to exclude signal fromchannels leading to said transducing means.

8. A combination according to claim 1 in which said direct currentbiasing means includes a player-operated key-switch; a timed momentaryimpulse relay activated by said key-switch; connections for deliveringthe timed momentary im-pulse from said relay to said gating circuit toestablish the delivered signal; and a continuously operative restoringconnection operative'ly associated with said gating circuit and adaptedto restore the initial inoperative condition of said gatin currentslowly, over an extended time period corresponding to the decay periodof a percussion instrument.

9. A combination according to claim 8 in combination with a secondrestoring circuit rendered operative by opening said keys-switch andoperatively associated with said gating circuit, for restoring theinoperative condition of said gating circuit more rapidly.

it). A combination according to claim 9 in combination withplayer-controlled means operatively associated with said secondrestoring circuit for eiiectuating a pedal sustain efiect by renderingsaid second restoring circuit inoperative regardless of the position ofsaid key-switch.

11. A gating circuit for electronic musical instruments comprising, incombination: terminals for receiving audiofrequency signal; a pluralityof diode attenuator sections connected to receive said signalseriat-irn; said attenuator sections being responsive to direct currentbias current and having high attenuation when biased, and relativelyinsignificant attenuation when not biased; a source of direct currentbias current operatively associated with said sections; a circuit fromsaid direct current source through a first resistor, and a secondresistor, to said attenuator sections; a capacitor connected into saidcircuit between said second resistor and said attenuator sections; alateral connection connected into said circuit between said first andsecond resistons; a first playing key in said lateral connection forclosing said connection to remove bias, and opening the same to permitbias to be restored; whereby the attack of the signal when said lateralconnection is closed is a function of the values of said capacitor andsaid second resistor, and the decay of signal when said lateralconnection is opened is a function of the value of said capacitor andthe combined impedances of both said first and second resistors; andconnections operatively associated with the last attenuator section fordelivering an output signal suitable for conversion into acousticaloscillations.

1 2. A gating circuit for electronic musical instruments comprising, incombination: terminals for receiving audiofrequency signal; anattenuator connected to receive said signal; said attenuator comprisinga signal-voltage divider having a relatively fixed impedance and ahighly variable impedance sensitive to direct current bias; a source ofdirect current bias current; a key switch and circuit for deliveringdirect current bias current from said bias source to said variableimpedance; said circuit comprising a connection from said source throughfirst and second resistors in series, to said variable impedance; afirst branch connection to ground drom between said resistors; said keyswitch being in said first branch connection; a second branch connectionto ground from between said divider and said second resistor;energy-storage means in the nature of a capacitor in said second branchconnection; whereby said energy storage means changes its condition bycurrent flow through said second resistor when said key switch isclosed, to determine the attack envelope of the attenuated signal, andchanges in the other direction by current flow through both resistors todetermine a longer decay envelope for the decay of the signal; andconections operatively associated with said attenuator for receiving theattenuated signal from said attenuator and delivering it to anelectronic musical instrument.

13. In an electronic musical instrument, in combination: a source ofaudio-frequency signal; a gating circuit connected to receive sign-a1from said source; said gating circuit having a first, operative, oropen, condition for passing signal; and a second, inoperative, orclosed, condition for preventing signal from passing through; one ofsaid conditions being identified as biased and the other as unbiased;transducing means for receiving signal from said gating means andgenerating cor-responding acoustical oscillation; rbiasing means forchanging said gating circuit from unbiased condition to biasedcondition; and playercont-rolled key-switch means for rendering saidbiasing means operative or inoperative; said biasing means comprising asource of direct current; a biasing circuit from the ungrounded terminalof said direct current source through a first resistor and a secondresistor to said gating circuit; a first lateral connection from saidbiasing circuit between said second resistor and said gating circuit; acapacitor in said first lateral connection; and a second lateralconnection from said biasing circuit between said first and secondresistors; said player-controlled keyswitch means being in said secondlateral connection.

14. In an electronic musical instrument, in combination: a source ofaudio-frequency signal; a gating circuit connected to receive the signalfrom said source; said gating circuit including aplurality of attenuatorsections, a first one of which receives the signal from said source, andeach subsequent one of which receives signal from a preceding section;each attenuator section including a diode and a resistor in series toact as a signal-voltage divider; each diode having a biased conditionand an unbiased condition; each diode, in one condition, having animpedance many times greater than its associated resister, and in theother condition, having an impedance many times less than said resistor;a source of direct current bias current operatively associated with saidattenuator sections; a playing key and a playing key-switch actuated bysaid key operatively associated with said source of direct current biaspotential; electrical connections operatively associated with saiddirect current source and with said key switch for selectivelydelivering bias current simultaneously to all said diodes; transducingmeans; and connections operatively associated with the last of saidattenuator sections for delivering signal from the last of saidattenuator sections to said transducin g means.

15. A combination according to claim 14 in which said diodes are allconnected in parallel with respect to direct current.

16. A combination according to claim 15 in which said diodes are two innumber, connected in parallel with respect to signal, but in series withrespect to direct current biasing current,

17. In an electronic instrument of the type comprising constantlyoscillating signal sources; a gating circuit comprising a firstattenuator section having a first resistor and a first variablecurrent-sensitive impedance in series with respect to signal; a secondattenuator section connected to receive the signal across said firstcurrent-sensitive impedance; said second similar attenuator sectioncomprising a second resistor and a second variable currentsensitiveimpedance; transducer means; connections for delivering the signalacross said second current-sensitive impedance to said transducer means;and connections for delivering direct current biasing current throughsaid first and second variable impedances in series.

18. A gating circuit for electronic musical instruments comprising, incombination: a first, shunt, signal-voltage divider comprising a firstresistor and a first diode; a second, series signal-voltage dividercomprising a second diode and a second resistor connected in series;said second divider being connected to receive the signal across saidfirst diode; connections for delivering the signal across said secondresistor; direct current bias means operatively associated with saidfirst and second diodes for keeping said first diode of low impedanceand said second diode of high impedance, whereby no signal is delivered;player-controlled key-switch means operatively associated with saiddirect current bias means for rendering said direct current bias meansinoperative, to permit said gating circuit to deliver signal; andconnections opcratively associated with said second, seriessignal-voltage divider for receiving the attenuated signal from saiddivider and delivering it as an output signal.

19. A combination according to claim 18 in which the signal across saidsecond resistor is subjected to formant means in the nature of a tunedcircuit adapted to produce damped oscillations at a frequency other thanthe frequency of the original signal.

20. A combination according to claim 18, in combination withenergy-storage means in the nature of a capacitor receiving thepotential coming to said second diode; whereby sharp discontinuitiescorresponding to undesirably high overtones are eliminated by dynamicchanges in the stored energy and in the dynamic impedance of said seconddiode.

References Cited in the file of this patent UNITED STATES PATENTS2,841,043 Sehreiber July 1, 1958 FOREIGN PATENTS 613,812 Great BritainDec. 3, 1948 622,910 Great Britain May 10, 1949 727,986 Great BritainApr. 13, 1955

1. IN AN ELECTRONIC ORGAN, IN COMBINATION: A SOURCE OF AUDIO-FREQUENCYELECTRONIC SIGNAL; A GATING CIRCUIT; SAID GATING CIRCUIT COMPRISING AFIRST ATTENUATOR SECTION COMPRISING A FIRST SIGNAL-VOLTAGE DIVIDER; SAIDDIVIDER COMPRISING A RELATIVELY CONSTANT IMPEDANCE AND ACURRENT-SENSITIVE VARIABLE IMPEDANCE; SAID DIVIDER HAVING AN INPUTTERMINAL CONNECTED TO RECEIVE SIGNAL FROM SAID SOURCE; A CONNECTION FORRECEIVING A FIRST DERIVED SIGNAL FROM ACROSS SAID VARIABLE IMPEDANCE; ASECOND ATTENUATOR SECTION COMPRISING A SECOND SIMILAR SIGNAL-VOLTAGEDIVIDER CONNECTED TO RECEIVE SAID FIRST DERIVED SIGNAL, AND COMPRISING ARELATIVELY CONSTANT IMPEDANCE AND A VARIABLE IMPEDANCE; A SECONDCONNECTION FOR RECEIVING A SECOND DERIVED SIGNAL FROM ACROSS THEVARIABLE IMPEDANCE OF SAID SECOND SIGNALVOLTAGE DIVIDER; TRANSDUCINGMEANS CONNECTED TO RECEIVE SAID SECOND DERIVED SIGNAL AND GENERATE SOUNDCORRESPONDING THERETO; AND DIRECT CURRENT BIASING MEANS OPERATIVELYASSOCIATED WITH SAID VARIABLE IMPEDANCES FOR CHANGING THE CONDITION OFSAID VARIABLE IMPEDANCES FROM HIGH TO LOW AND VICE VERSA.